On Wed, 9 Jun 2021 at 17:33, Segher Boessenkool
[off-list ref] wrote:
[...]

quoted

An alternative design would be to use a statement attribute to only
enforce (C) ("__attribute__((mustcontrol))" ?).

Statement attributes only exist for empty statements.  It is unclear how
(and if!) we could support it for general statements.

Statement attributes can apply to anything -- Clang has had them apply
to non-empty statements for a while. I have
[[clang::mustcontrol]]/__attribute__((mustcontrol)) working, but of
course it's not final but helped me figure out how feasible it is
without running in circles here -- proof here:
https://reviews.llvm.org/D103958

If [1] is up-to-date, then yes, I can see that GCC currently only
supports empty statement attributes, but Clang isn't limited to empty
[2].
[1] https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html
[2] https://clang.llvm.org/docs/AttributeReference.html#statement-attributes

In fact, since C++20 [3], GCC will have to support statement
attributes on non-empty statements, so presumably the parsing logic
should already be there.
[3] https://en.cppreference.com/w/cpp/language/attributes/likely

Some new builtin seems to fit the requirements better?  I haven't looked
too closely though.

I had a longer discussion with someone offline about it, and the
problem with a builtin is similar to the "memory_order_consume
implementation problem" -- you might have an expression that uses the
builtin in some function without any control, and merely returns the
result of the expression as a result. If that function is in another
compilation unit, it then becomes difficult to propagate this
information without somehow making it part of the type system.
Therefore, by using a statement attribute on conditional control
statements, we do not even have this problem. It seems cleaner
syntactically than having a __builtin_() that is either approximate,
or gives an error if used in the wrong context.

Hence the suggestion for a very simple attribute, which also
side-steps this problem.

Thanks,
-- Marco